Effect of nickel on root growth and the kinetics of metal ions transport in onion (Allium cepa) root.

The effect of different concentrations of nickel nitrate (0.25, 0.50, 1.00 and 2.00 mM) uptake by the roots, on root growth of onion (Allium cepa) and the transport of Ni2+, Fe2+, Mn2+, Zn2+, K+, Na+ and H+ ions were investigated spectrophotometrically. The uptake of Ni2+, Fe+, Mn2+ and Zn2+ was monitored by flame atomic absorption spectrometry with a 24-h period for 7 days and the amounts of K+ and Na+ were determined in solutions by flame photometer. The mineral content of the solution, instead of the root material was measured. Ni2+ ions showed inhibitory effect on the root growth at all concentrations during the entire treatment. The ECso (effective concentration that reduced root growth by 50%) was found at 0.25 mM Ni2+. No significant change in inhibitory effect was observed after at 0.50 mM Ni2+ concentrations. A large amount of Ni2+ was translocated into the roots. The kinetics of metal ion transport followed a pseudo-first order reaction in all metal ion concentrations. Ni2+, Zn2+ Fe2+ Mn2+ and H+ ions transferred together into plant, but Na+ and K+ ions transferred to the solution from the plant. The amount of H+ in the solution decreased at all Ni2+ concentrations.